Well, if you are already using scope rings to support the focuser/drawtube then that should help. But, if the rings are just supporting the optical tube then you still need to be concerned about flexure in the focuser/drawtube itself (the loose slip fit between the tube and the focuser housing could be causing problems). I don't know which setup you are using. Rings can hold the optical tube fairly steady, but that alone won't prevent the drawtube from moving.
I'll take some pictures of the ST80 setup. The rings currently just hold the optical tube. I hadn't thought of using the rings to hold the drawtube...I guess I could get a third smaller ring to do that. The focuser has a locking knob that presses on the drawtube. When I tighten it down, it does change the framing a bit, but it eliminates most of the play in the drawtube. It doesn't eliminate all of it, and I understand that all it takes is a fraction of a millimeter to cause problems.
As for the Orion mini-guider, it's not perfect but my major concern with it would be the image scale it produces with your Orion Starshoot Guider. I mentioned early on that it was probably nearing the boundary that I would consider safe for an autoguider image scale (1:5 with your setup), but that it was probably not the major source of your problems. To repeat from earlier, Craig Stark says that the star centroid calculations in PHD can determine a star's center to an accuracy of 1/5 pixel under even very noisy conditions (or much better under less noisy conditions, better seeing, better guider, etc.).
However, there are some in the industry who doubt that the mathematical simulations that are being used to PREDICT such centroid accuracy are really that good in the actual field. I guess one "rule" that has been kicked around is a 1:30 scale, but given the above I find that perhaps very optimistic. So, YMMV when moving much beyond that 1/5 pixel accuracy (IMO).
Aye! This is what you mentioned on my previous thread on this topic. My ratio with the 50mm guidescope is 1:4.5. So, while it isn't beyond that 1:5 ratio that Stark mentions, it's pretty close. At best, I was able to get about 4-5" P2P tracking with the 50mm, with RMS on average around 2", but as high as 3". At the absolute best, when pointed closer to the meridian, with everything tuned about as optimally as I currently have the skill to tune it, the 50mm guidescope and SSAG got as low as 1.5" RMS, however my P2P error was still about 4", so while guiding seemed a bit smoother, the effect on my star size and quality did not really change much.
With the ST80, the ratio is 1:1.8. If I can resolve my flexure issues, I think I can take my guiding performance much farther with the ST80 than I can with the 50mm mini guidescope. It was this 1:5 ratio thing, along with the mention in the previous thread that the 50mm mini guidescope had flexure issues of its own, that made me buy the ST80.
I went back and looked at your most recent unguided PHD graphs and I see two things. First, it appears that you have an error in one axis of your polar alignment. I say this because your drift lines are much better on the meridian than they are toward the west. Specifically take notice of this statement from the following document/link:
Notice that the optimal point for observing azimuth error is at the intersection of the equator and the meridian. But due to the orthogonal geometry this is also the location where altitude error is virtually undetectable. Conversely, we observe altitude errors best on the equator at the eastern or western horizon, and in this location azimuth error is minimized. This is how we are able to use the drift method to measure and correct azimuth and altitude errors independently. Of course, this all assumes that the mount base is leveled, as an unleveled base would cause azimuth corrections to affect altitude and vice versa.
Interesting. Just to make sure you are fully aware (I'm not sure if this changes anything or not), but my unguided PHD altitude graph was not exactly "near the horizon". I have homes and trees in the way of that. I think I was probably about 45° up from the horizon. I do agree, there is more error in altitude than azimuth. I'm a bit wary of tweaking altitude, because there is no smooth way to do it. The weight of everything on the mount, which ultimately sits directly on that one L-bolt for altitude adjustment, makes it difficult to make fine adjustments. Maybe it's just a skill that you learn with practice, but usually when I try to adjust altitude, I end up making short jumps in movement, which results in the alignment swinging even farther out, rather than tighter in, and the night usually devolves into altitude L-bolt tweaking instead of imaging.
PHD shows my altitude error to be pretty low, though...0.29', or 17.4". Is that enough to be concerned about?
Secondly, there seems to be a periodic shift in declination that follows the PE in R.A (on the unguided graph taken near to the meridian). I see this same issue on my Celestron AVX mount and I THINK this is being caused by a wobble in the polar axis that happens as the R.A. worm gear moves against the polar spur gear. This could happen if there is any play in the bearing on the polar axis and if the R.A. worm and spur gear are not in perfect alignment (among other possibilities). It could also be affected (made worse or better) depending upon the balance of the mount.
Aye! This is the declination oscillation issue. I really don't know what causes it, any insight into what might is extremely welcome. It's been another thorn in my side, and has made it practically impossible to guide in dec because dec drift will periodically swing to the non-guided side of the midline in PHD, and then he potential error is uncontrolled...it often gets as high as 3-4" off the midline before finally drifting back down. I'd tried guiding dec in both directions, and of course backlash then becomes the problem, and longer and longer guide pulses eventually send declination racing to the opposite side, and you get a faster and more pronounced oscillation.
It's an infuriating problem, however it doesn't affect my stars as much as the flexure issue does, so I've just lived with it for now. And, rather ironically, when my declination plot is smooth, the elongation of the stars in RA tends to be much more stark and less easy to work around...so as long as the unguided swings don't top 2", I've usually welcomes it.
I'd be interested if anyone else has experienced such an issue and what they think is causing the APPARENT correspondence between the PE and a similar period/cycle of movements on the declination axis (which when unguided should not be moving).
I too would be interested in hearing anyone else's experience with this.